Essential to any optical telecommunication system are optical assembles for switching, filtering, and multiplexing/demultiplexing optical signals. For example, the use of wavelength division multiplexing (WDM) techniques has increased significantly the transmission capacity of fiber-optic communication systems. In a WDM communication system, multiple channels, in which each channel is differentiated by using a unique wavelength of light, carry modulated optical signals in a single optical fiber between a transmitter and a receiver. The transmitter uses an optical multiplexer to combine multiple channels into the fiber for transmission, and the receiver uses an optical demultiplexer to separate the optical channels for detection. Multiplexers and demultiplexers therefore are essential for the high transmission capacity of state-of-the-art optical systems.
Of particular interest herein are optical assemblies which rely on reflective elements to perform their specific functions. As used herein, the term “optical device” refers to such an optical assembly which uses reflective elements for the discretionary treatment of channels of an optical beam. Examples of discretionary treatment of channels include switching, filtering and multiplexing/demultiplexing channels.
Demands on these optical devices have increased dramatically in recent years as the need for higher transmission capacity in optical systems has increased. In particular, this need has lead to recent innovations in switching techniques such as those disclosed in US Patent Application No. 20020196520, which is hereby incorporated by reference. That application discloses, among other embodiments, a programmable optical multiplexer/demultiplexer. In this device, the WDM channels enter the switch from the central fiber, the light coupled out from the fiber is collimated and imaged on a diffraction grating. The light diffracted from the grating is spectrally separated and focused on an array of micromirrors which are controlled by a micro electromechanical system (MEMS). Each channel of the WDM signal is reflected by a different micromirror. By actuating a particular mirror, a particular channel may be coupled to or decoupled from a particular output fiber. In this way, every wavelength of the WDM signal input can be distributed to any desired output fiber.
Although this development provides for a highly configurable WDM device, the device nevertheless must be designed for channels having particular bandwidths, spacing, and wavelengths. This presents several problems. First, the micromirrors of the OADM must be designed for a specific target bandwidth. Specifically, the spacing of the micromirrors is chosen to map the desired ITU wavelength grid on the mirror center. Because the position of the light spots is linearly spaced in wavelength on the micromirrors and the WDM signals are equally spaced in frequency, the spacing of the mirror is changing across the linear MEMS array. The micromirror design will therefore be a function of both the wavelength band of the WDM signal and the diffraction grating period. The channel frequencies of the device must therefore be precisely aligned with the channel wavelength of the WDM carriers frequencies, requiring a redesign of the microstructure for every different channel configuration. This hardware-specific design prevents the device from being reconfigured as data rates increase. Hence, the device is not upgradable.
Second, since the micromirror must be precisely tailored for particular channels, its positioning within the optical device must be equally precise, typically within 1–5 μm. Such precision is difficult to achieve and requires active alignment of the micromirror position. It is well known that active alignment is an expensive and time-consuming process that hinders large-scale manufacturing.
Therefore, there is a need for an optical device which is not designed for particular channel parameters and thus can adapt to changing channel bandwidths, spacing, and wavelengths as the optical system evolves. There is also a need for an optical device which does not require active alignment of its components and which lends itself to large-scale manufacturing. The present invention fulfills these needs among others.